1. Field of the Invention
This invention relates to a process for the preparation of optionally coupled, block-free polymers based on conjugated dienes and optionally monovinylaromatic compounds, the products of the process, and their use for producing tires and antivibration elements. The products of the process are distinguished by a high proportion of 1,2-structural units of butadiene and 1,2- and 3,4-structural units of isoprene.
2. Description of the Background
Numerous processes are known for preparing conjugated diene polymers. The processes in which the monomers are polymerized anionically in an inert solvent such as hexane, cyclohexane, or toluene especially have gained industrial importance. Organoalkali metal compounds, especially alkyllithium compounds, are used as initiators, which are usually called catalysts.
Rubbers with star-shaped structures have advantages over their homologs with linear structures (cf. European Patent Application Disclosure 0 090 365). They are obtained by converting the diene monomers into so-called "living polymers" and subsequently reacting the living polymers with couplers such as polyhalo compounds or divinylaromatic compounds.
It is known that the presence of certain polar compounds, which are also called cocatalysts, has an effect on the microstructure of the diene polymers and thus on the course of the polymerization with regard to preferential formation of 1,2- and 3,4-structural units (cf. K. H. Nordsiek, K. M. Kiepert, Kautschuk and Gummi, Kunststoffe 35, 371 (1982), and K. H. Nordsiek, Kautschuk and Gummi, Kunststoffe 39, 599 (1986).
A number of processes for preparing polymers based on conjugated dienes have been developed in the past in which various cocatalysts have been used. It has been found that the cocatalyst affects the polymerization in many ways. The following requirements must be placed on the cocatalyst:
1. It should not lead to any odor contamination. This means, for example, that tertiary amines, which have also been proposed as cocatalysts, are unsuitable for this reason. In addition, amines have an undesirable affect on the adhesion of tires to steel.
It should control the course of polymerization so that the polymers obtained have more than 70% 1,2- and 3,4-isoprene structural units and more than 60% 1,2-butadiene structural units.
3. Basically, the effect of a cocatalyst depends on the ratio of the molar quantities of cocatalyst and catalyst, where the catalyst is the organolithium compound. The control effect desired as stated in paragraph 2) above should be achievable even with a cocatalyst/catalyst molar ratio of 10:1.
4. The addition of a cocatalyst affects the yield of polymer. Cocatalysts with which yields of 90% cannot be obtained are unsuitable.
5. The cocatalyst should be largely inert to the "living polymer" present as anion during the polymerization at temperatures up to 125.degree. C. This requirement is particularly important when the living polymers are reacted with coupling agents to form star-shaped rubbers, or with suitable electrophilic compounds, after completion of the polymerization.
6. The cocatalyst should be easy to separate from the solvent by distillation. 7. The cocatalyst should be readily available synthetically.
The only compounds known to come close to meeting the above requirements are those which contain ether groups. Aliphatic dialkyl ethers such as diethyl ether and cyclic ethers such as tetrahydrofuran (THF), when employed as cocatalysts, do not exert adequate control on the microstructure of the polymer (cf. T. A. Antkowiak et al., J. of Polymer Science Part A-1, Vol. 10, pages 1319 to 1334 (1972)). Thus, for example, in the anionic polymerization of butadiene with butyllithium in the presence of up to even an 85-fold excess of THF, a polybutadiene is produced that contains only 49% 1,2structural units. The need to use such a large amount of cocatalyst is a significant drawback to its use in practice.
The same literature reference shows that in the polymerization of butadiene with ethylene glycol dimethyl ether as cocatalyst, a polymer can be obtained which contains 63.7% 1,2-structural units. However, this glycol ether has two substantial drawbacks. In the first place, it cannot be separated from hexane directly. In the second place, the coupling yield of this ethylene glycol dialkyl ether is 0%. It is therefore assumed that the two other representatives of this class of compounds known in the art, namely ethylene glycol diethyl ether and ethylene glycol dibutyl ether, would not meet the above requirements, either.
Besides the ethylene glycol dialkyl ethers, diethylene glycol dialkyl ethers have also been proposed as cocatalysts (for example, see European Patent Application Disclosure 0 090 365). The results obtained here are also unsatisfactory. In the case of the butadiene-isoprene copolymers disclosed by Japanese Patent Application Disclosure 82/87 406, which are obtained by coplymerization in the presence of butyllithium and diethylene glycol dimethyl ether, the conversion is unsatisfactory. When diethylene glycol dimethyl ether is used in the polymerization of butadiene and styrene, a coupling yield of 0% is obtained (cf. Example 8 of U.S. Pat. No. 4 530 985). Independent experiments with this glycol ether in the polymerization of isoprene gave a polymer yield of only 78%.
In experiments with diethylene glycol diethyl ether or ethylene glycol diethyl ether where isoprene is polymerized, improved yields of polymer are obtained; however, it has not been possible to obtain star-shaped rubbers. This is evidence that no living polymers at all were present in the reaction medium at the time of addition of the coupling agents. As stated in U.S. Pat. No. 4,530,985, patentee logically concluded that "many of the modifiers (meaning cocatalysts) that have generally been proposed in the past, such as diethylene glycol dimethyl ether, tend to terminate the polymerization reaction prematurely and/or to interfere with the coupling reaction" (cf. Column 1, lines 31 to 35).
Kminek et al. have also come to the conclusion that the "living polymers" are stable only for a short time in the presence of ethylene glycol dimethyl ether (cf. Makromol. Chemie 183, 1755 (1982).
European Patent Application Disclosure 0 128 607 discloses a process for the preparation of copolymers of an aromatic vinyl compound and a conjugated diene. The copolymer obtained has a vinyl content of at least 30 wt. %, is characterized by a particularly high proportion of aromatic vinyl compounds in the initial and terminal sections of the polymer chain, and is prepared by polymerization of monomer in the presence of a microstructure modifier of the general formula: EQU R.sup.1 --O--CH.sub.2 --CH.sub.2 --O--R.sup.2
in which R.sup.1 and R.sup.2 are identical or different alkyl groups containing from 2 to 18, especially 2 to 6 carbon atoms. U.S. Pat. No. 4,530,985 proposes microstructure modifiers of the general formula: EQU R.sub.1 --O--CH.sub.2 --CH(R.sub.2)--O--CR.sub.3 R.sub.4 --O--R.sub.5
instead of the conventional glycol ethers. The microstructure control and the coupling yield using these cocatalysts are also unsatisfactory. Moreover, the compounds are difficult to obtain.
Unpublished German Patent Application P 37 07 434.2 discloses an anionic polymerization process in which an organolithium compound is used as the catalyst and an ethylene glycol dialkyl ether as the cocataIyst of the formula R.sub.1 --O--CH.sub.2 --CH.sub.2 --O--R.sub.2, wherein R.sub.1 and R.sub.2 can be C(1-4) alkyl groups and the groups R.sub.1 and R.sub.2 can have different numbers of carbon atoms. However, this process is limited to the polymerization of isoprene. In the light of what is known in the art, no process is known for the preparation of polymers from at least two monomers selected from the group consisting of butadiene, isoprene, and styrene, in which the practical requirements mentioned here for the cocatalyst are met. A need therefore continues to exist for a method of producing a block-free conjugated diene polymer of improved characteristics, in a manner which is industrially more acceptable.